Tanks for the bulk transport and/or storage of semisolid and liquid materials of the kind comprising a tank with a movable piston therein are well known in the art. Examples of such tanks may be found in U.S. Pat. Nos. 3,828,988; 4,721,235; 5,114,054; and 5,341,726. The tanks usually have follower pistons with pneumatically expandable seals surrounding one end of the tank for seating the piston relative to the tank to accommodate changes in the interior cross-section of the tank. The seal is generally positioned between circumferential flanges affixed to the outer surface of the tank in order to axially retain the seal during movement of the piston. The tanks also generally have pads positioned about the piston and extending radially outwardly therefrom for preventing canting of the piston as the piston moves within the tank.
As indicated above, the prior art seals are usually hollow and capable of being filled with air to cause the seals to expand. The hollow portion or chamber of the seal may be filled and depleted of air through a valve structure in communication with the chamber and disposed within the piston. The valve is accessible through a rear opening in the piston. Since the seal is naturally between the outer surface of the piston and the inner surface of the tank, and is in contact with the inner surface of the tank as the piston reciprocates within the tank, the seal is subject to abrasion and wear. Also, because the seal is pneumatic, there is always the possibility that a puncture will develop and render the seal useless. Thus, although pneumatic seals are efficient, they are prone to failure. Furthermore, the friction created on the seal by the reciprocating piston may occasionally cause the seal to roll out of position. Additionally, pneumatic seals are difficult to fasten securely to the piston because they cannot be punctured by a fastener.
It has been suggested that the design of the seal be such as to effect a wiping action against the inner surface of the tank. This however, is subjecting the seal to more wear and exposing the seal to a greater possibility of failure.
In view of the above, it is an object of the present invention to provide an improved seal structure for a tank piston.
It is another object of the present invention to provide a piston seal that will accommodate expansion while providing a longer wear life.
It is yet another object of the present invention to provide a tank piston structure that includes an improved wiper structure.
It is still another object of the present invention to provide a tank piston that has an improved wiper structure for the inner surface of the tank, and having an improved seal structure that moves along the inner surface of the tank with less friction.